Sam Haggerty singled with two outs in the ninth inning and hurt his left groin while sliding into second base on a successful steal. He needed assistance to leave the field, an arm draped over manager Scott Servais and an athletic trainer.
Haggerty is scheduled for an MRI on Tuesday.
"Getting to the end of the regular season is always a challenge,'' a reserved Servais said.
"With everything that happened today, we're not going to be able to host a playoff game -- in the first series -- here at T-Mobile Park, so now we've got to adjust a little bit," Servais said. "The focus right now is trying to get as healthy as we can, as rested as we can, before we start our series here on Friday, and that will be challenging."
Part of that plan came together as Seattle star Julio Rodriguez, back from a trip to the injured list caused by a lower back strain, was the designated hitter and likely will be in center field for one of the games in Tuesday's doubleheader. The rookie hit a 110 mph single up the middle leading off the first, advanced when Ty France was hit by a pitch and tied the score at 1 when he moved up on flyouts by Mitch Haniger and Eugenio Suarez.
Rodriguez dropped another hit between three fielders in the third inning and cut the lead to 4-3 with an RBI single in the seventh against Jose Cisnero.
Haggerty's injury cast a bit of a pall over the Mariners after a weekend of celebration since the team clinched a wild-card berth Friday night.
"Haggs has had such a great season,'' Servais said. "Kind of like that sparkplug guy, you fire him in there and he makes things happen. And sure enough, he gets a big hit late in the game tonight. He got a great jump. He just kind of let his guard down. I think he thought he had the base stolen pretty easy, and he did, he just waited too late to slide and caught the bag wrong.''
Seattle rookie George Kirby (8-5), bidding for a postseason start, allowed four runs, six hits and three walks in four innings. Kirby entered with the highest strikeout-to-walk ratio by a rookie since 1900 at 6.74.
"Tonight was just not a typical night we've seen from George Kirby really all year,'' Servais said. "Just not real sharp. He walked more guys than he normally does in a month in one game tonight.''
Seattle optioned infielder Abraham Toro to Triple-A Tacoma earlier Monday. Toro is expected to join a group of players in the Mariners' system currently working out in Arizona in case they are needed during the playoffs. Seattle also reinstated catcher Curt Casali from the paternity list and optioned catcher Brian O'Keefe to Tacoma.
Luis Castillo is in line to start the wild-card opener for Seattle.
"He's got all the intangibles, plus the stuff, and he likes pitching in big games and this is going to be a big game,'' Servais said. "I say all that and we can't get too far down the road. We've got four games left to play.''
The Associated Press contributed to this report.